JP2024501614A - Ultra large mode area single mode amplification optical fiber and fiber amplifier or laser incorporating it - Google Patents

Abstract

The present invention relates to an ultra large mode area single mode amplification optical fiber (100), the ultra large mode area single mode amplification optical fiber (100) includes a doped core (1) with a core diameter larger than 20 micrometers, and the doped core (1) A solid matrix made of a first glass and surrounded by at least a first cladding (2) comprising two stress-applying parts (21, 22) arranged symmetrically with respect to the core (1), The applicators (21, 22) are aligned along the alignment axis (20) and the cladding has two flat surfaces (4, 14) extending parallel to the longitudinal axis (10) and transversely to the alignment axis (20). ), the two flat surfaces (4, 14) are coupled by two curved surfaces (5, 15), and the optical fiber (100) is connected to the plane (30) making an angle of less than 15 degrees with the alignment axis (20). ) with a bending diameter of less than 30 cm, while the bending loss is 0.5 dB/m.

Description

The present invention relates to single mode amplifying optical fibers, amplifier fibers or laser fibers for generating peak high power radiation with high spatial quality.

More precisely, the present invention relates to fibers and methods of manipulating and manufacturing said optical fibers.

Over the past 15 years, the power of fiber lasers and/or fiber amplifiers for producing continuous and/or pulsed laser radiation has increased dramatically. This is due in part to the design of the optical fiber, which is doped with rare earth elements, which provides an amplification medium with lengths on the order of meters, while also allowing heat dissipation along the fiber. Furthermore, the development of fiber amplifiers based on large mode area fibers has led to the development of four-wave mixing (FWM), self-phase modulation (SPM), stimulated Raman scattering (SRS), etc. By limiting the undesirable nonlinear optical effects of , this resulted in a significant increase in peak power. In fact, such nonlinear optical effects tend to induce spatial and temporal distortions within the amplified pulse of light. These large mode area fibers allow single mode propagation over a large cross section.

The manufacture of single-mode optical fibers is an important parameter for delivering a laser beam with high spatial quality, i.e. with a radiation quality factor M ² of the laser beam lower than 1.05 or as close to 1 as possible. If the fiber is multimode rather than single mode, the radiation quality will be degraded. Only single-mode fibers, or in other words fibers with one transversely propagating mode, can provide the required laser beam quality.

Various documents describe apparatus and methods for manufacturing large mode area single mode amplifying optical fibers.

A large mode area fiber (or LMA fiber) is defined herein as an optical fiber whose effective area in the fundamental mode (denoted by _Aeff ) is greater than approximately ^90λ2 , where λ is guided within the fiber. , is the wavelength of the signal being amplified. Effective area is usually expressed as:
where E is the spatial envelope of the electric field of the mode, and the integral is generally computed over the circular lateral area of the fiber. For approximately Gaussian shaped modes, the mode field diameter (i.e. MFD) is expressed as:
where r represents the radial coordinate. For pure Gaussian modes, the effective area is related to the mode field diameter by the following expression: A _eff = π×MFD ² /4. As used herein, the term <<mode>> refers to a transverse mode of an electromagnetic wave, i.e. an optical signal propagating in a fiber, which may include an amplifier or, in the case of a laser, an amplified or guided signal. Reference herein to propagation of light in a single mode is intended to include propagation in a single transverse mode that is approximately Gaussian in nature.

More recently, ultra-large mode area fibers are now being developed all over the world. A very large mode area fiber (or VLMA fiber) is defined as an optical fiber whose effective area in the fundamental mode is greater than about 375λ ² , where λ is the wavelength of the signal.

Different optical fiber designs have also been proposed, particularly for producing VLMA amplification fibers.

The first approach is based on a conventional step index fiber that includes a solid core and a solid first cladding that has a higher refractive index than the core. The core is typically doped with rare earth ions to amplify optical radiation. In conventional fibers, signals coupled into the core propagate by total internal reflection due to the difference in refractive index between the doped core and the cladding. However, manufacturing conventional large mode area fibers becomes more difficult as the effective area increases. Furthermore, when the core diameter is increased to obtain a large mode area, the fiber becomes multi-mode, resulting in propagation in higher-order modes. Various refractive index profiles for the core have also been proposed, such as step index, flat top, or parabolic profiles. It is also possible to reduce the numerical aperture (NA) to support fewer modes. However, decreasing the NA makes the fiber more difficult to manufacture. Furthermore, a small NA results in high bending losses at bend radii smaller than 30 cm. Due to current industry manufacturing process limitations, it is difficult to manufacture essentially single mode conventional step index fibers with core diameters greater than 20 micrometers (μm). Additionally, when obtaining a compact system by winding large mode area fibers, higher order mode (HOM) coupling and bending losses are induced. The patent documents US 2009/262761 and US 2010/195194 disclose panda-type VLMA amplification optical fibers.

Other approaches are based on the use of microstructured fibers or photonic crystal fibers (PCFs) that include a core and a surrounding array of voids or doped silica inclusions. Similar to conventional fibers, signals propagate by total internal reflection due to the refractive index difference between the average refractive index of the microstructured cladding and the central solid core. In theory, very large mode areas can be obtained in these fibers, the numerical aperture of which is very low depending on the size of the pores or added silica inclusions. However, microstructured fibers and PCFs are very susceptible to bending. Large mode area doped microstructure fibers have other disadvantages, such as manufacturing complexity, cost, and difficulty in fiber handling, such as cutting and splicing. Winding a microstructured fiber can induce losses and reduce the effective mode area compared to the same fiber design in the straight state. Therefore, the core diameter is generally limited to about 40 μm and the PCF fiber is preferably used in a straight state, which in practice provides a fiber length that is compatible with compactness requirements, especially for laser systems. is limited. Additionally, higher concentrations of aluminum oxide (Al ₂ O ₃ ) and/or phosphorus pentoxide (P ₂ O ₅ ) co-dopants are required, and optical darkening and clustering effects, which degrade the short- and long-term performance of fiber amplifiers, can be introduced. To prevent adverse effects, the fiber is doped with rare earth ions. In addition, PCF designs require approximately equal or slightly lower silica core refractive index, which limits the incorporation of co-dopants and/or increases the concentration of rare earth ions to an extent that no adverse effects occur. .

Yet another approach is based on the use of polarization-maintaining ytterbium-doped fibers (PM-YDF) based on PCFs with very large holes. X. Peng an L. Dong (“Fundamental mode operation in polarization-maintaining ytterbium-doped fiber with an effective area of 1400μm ² ”Op t. Lett. Vol. 32, no. 4, Feb. 15, 2007 pp. 658-360) has a diameter of approximately 50 μm discloses a PM-YDF comprising two boron-doped stress elements and four holes around a core of. This fiber has a critical bending radius of 4 cm in the fundamental mode and operates in single mode when the hole dimensions are precisely designed to increase leakage losses in higher order modes. Similar to PCF fibers, the doped region of the core exhibits a refractive index only slightly lower than that of silica, by 2.0×10 ⁻⁴ .

An alternative approach is to commercialize polarized PCF fibers or microstructured fibers that inherently support several modes, which are forced to operate in single mode through bending with a bend radius of less than 40 cm. However, the effective area of these fibers cannot be scaled arbitrarily with the size of the fiber core. Furthermore, the output power decreases when the bending diameter becomes less than 30 cm.

Currently, the maximum power available is 1500 W in continuous wave operation using PM fiber.

However, as the core diameter increases, it becomes increasingly difficult to achieve single mode operation.

There is a need for a fiber design that allows ultra-large mode area single mode amplifying fibers to be obtained at low manufacturing costs and that is easy to splice with conventional step index single mode fibers.

Accordingly, one object of the present invention is to provide an ultra-large mode area single mode amplification optical fiber.

The above object is achieved according to the invention by providing an optical fiber comprising a core extending along the longitudinal axis of the optical fiber, the core being solid, the core comprising at least an element providing one emission band is added, the core diameter of the core is greater than 30 μm, the core is surrounded by at least one glassy cladding including a first cladding, the first cladding being a first glass. The fabricated solid matrix includes two stress-applying parts (i.e., SAPs) arranged symmetrically about the core, the first glass having a lower refractive index than the core, and the core and the two stress-applying parts having a lower index of refraction than the core; aligned along an alignment axis transverse to the longitudinal axis, the at least one glassy cladding includes at its outer periphery two flat surfaces extending parallel to the longitudinal axis and transverse to the alignment axis; The flat surface is arranged symmetrically with respect to the core and is joined by two curved surfaces, the optical fiber being suitable for being bent with a bending radius of less than 30 cm in a plane containing the longitudinal axis of the fiber, said plane being It makes an angle of less than 15 degrees with the alignment axis and has a bending loss of less than 0.5 dB/m in the fundamental mode.

This optical fiber is adapted to amplify signals at wavelengths corresponding to the emission band of the ions. With this design and configuration, it is possible to obtain an amplifying optical fiber with an ultra-large mode area and operating in the single mode regime. Due to the two stress applying parts, this amplifying optical fiber is a polarization maintaining fiber. Additionally, when bending or winding the fiber, one flat surface mechanically guides the fiber to bend in a plane that makes an angle of less than 15 degrees with the alignment axis of the two stressors. This bending makes it possible to limit the losses induced in the fundamental mode while suppressing the higher order modes, thus allowing the fiber to amplify the fundamental mode in the single mode regime.

In fact, at certain bend diameters (less than 30 cm), the fiber exhibits losses in higher order modes (HOMs) of 10 dB/m or more and therefore operates in the single mode regime.

According to certain aspects of the present disclosure:
- the bending diameter is between 10 cm and 30 cm or between 10 cm and 20 cm and the bending loss is less than 0.5 dB/m in the fundamental mode;
- the optical fiber is bent in a plane making an angle of less than 10 degrees, preferably less than 5 degrees, with the alignment axis;
- the effective area of the fiber is greater than 375 μm ² , preferably greater than 450 μm ² at a signal wavelength suitable to be amplified by said fiber;
- the difference in refractive index between the core and the first glass is higher than 5.10 ^-4 ;
- the difference in refractive index between the core and the first glass is between ^5.10-4 and ^2.5.10-3 or between ^5.10-4 and ^1.10-3 ;
- the core and the first glass are based on silica glass, or on fluoride glass, or on chalcogenide glass, or on phosphate glass,
- The stress applying part is made of silica to which boron oxide, germanium oxide, fluorine oxide, or a co-dopant of phosphorus oxide and boron is added, or a co-dopant of aluminum oxide and phosphorus oxide, a co-dopant of phosphorus oxide and aluminum oxide, or the like. containing silica doped with a so-called doping combination that creates a stress region whose refractive index is lower than that of the first glass;
-Rare earth ions are added to the core,
- the rare earth ion is selected from any lanthanide ion that provides at least one emission band;
- the rare earth ion is selected from ytterbium, erbium, thulium, and holmium, or any combination thereof;
- the core is doped with an element that provides an emission band within the glass, such as chromium ions or other metal ions that provides at least one emission band between 800 nm and 2500 nm (nanometers);
- the core presents a flat top (stepped index fiber) or parabolic index profile;
- the core includes a pedestal surrounding a central portion of the core, the pedestal having a refractive index lower than the central portion of the core and higher than the first glass;
- the at least one glassy cladding consists of a first cladding, the first cladding comprising two flat surfaces at its outer periphery;
- the at least one glassy cladding includes a second cladding disposed around the first cladding, the second cladding having a refractive index lower than the first cladding;
- the second cladding is selected from an all-solid cladding made of a second glass or an air cladding and a solid cladding made of a second glass, the air cladding being made of the first cladding and a second glass; placed between the solid cladding and
- the fiber further comprises a polymer or metal cladding surrounding said at least one glassy cladding;

Another object of the invention is to provide a fiber amplifier comprising an ultra-large mode area single mode amplifying optical fiber according to any one of the disclosed embodiments, wherein the ultra large mode area single mode amplifying optical fiber is Wound around a coil or around a circular plate with a bending diameter of less than 30 cm.

According to certain aspects of fiber amplifiers according to the present disclosure:
- The length of the optical fiber is between 50cm and 20m,
- the bending diameter is between 10 cm and 25 cm, preferably between 15 cm and 20 cm;
- the optical fiber provides an effective area of greater than 375 μm ² , preferably greater than 450 μm ² ;
- the fiber amplifier comprises a pump light source for generating a pump beam and an optical beam combiner adapted to inject said pump beam into the core and/or into the first cladding;
- The fiber amplifier is configured to amplify a beam with a wavelength between 800 nm and 2500 nm, depending on the added element of the core.

Another object of the present invention is to provide a fiber laser comprising an ultra-large mode area single mode amplification optical fiber according to the present disclosure, wherein the ultra large mode area single mode amplification optical fiber is wound with a bend diameter of less than 30 cm. The fiber laser further includes a first mirror at a first end of the ultra-large mode area single mode amplification optical fiber and a second mirror at a second end of the ultra large mode area single mode amplification optical fiber.

Advantageously, the first mirror and/or the second mirror include a fiber Bragg grating.

DETAILED DESCRIPTION OF THE EMBODIMENTS The following description with reference to the accompanying drawings makes it clear what constitutes the invention and how it can be realized. The invention is not limited to the embodiments shown in the drawings. It should therefore be understood that, while features recited in the claims may be accompanied by reference signs, such signs are included only to facilitate an understanding of the claims and that the patent This does not limit the scope of the claims at all.

1 schematically illustrates in cross-section the design of a stepped index large mode area optical fiber and its geometric parameters according to the present disclosure; FIG. 2 schematically shows a first example of a refractive index profile along the alignment axis of a stressor for a single mode polarization maintaining fiber according to the present disclosure; FIG. 2 schematically shows a second example of a refractive index profile along the alignment axis for a single mode polarization maintaining fiber according to the present disclosure; 4 illustrates exemplary measurements of a refractive index profile for an optical fiber designed in accordance with the present disclosure and shown in cross-section in FIG. 4. FIG. 1 schematically shows a cross-sectional image taken by a scanning electron microscope of a step refractive index large mode area optical fiber according to a first embodiment of the present disclosure. Figure 2 shows measurements of the modal loss for bending the fiber as a function of the azimuthal angle θ between the bending plane and the alignment axis of the stressor, as schematically shown in Figure 1; Figure 3 shows measurements of modal loss of a VLMA fiber according to the present disclosure as a function of bend radius when the fiber is bent in a plane parallel to the alignment axis of the stressor. Figure 3 shows the measured modal loss of a VLMA fiber as a function of bend radius when the fiber is bent in a plane perpendicular to the alignment axis of the stressor. 2 schematically depicts in cross-section a variant or alternative embodiment of an optical fiber according to the present disclosure; FIG. 2 schematically depicts in cross-section a variant or alternative embodiment of an optical fiber according to the present disclosure; FIG. 2 schematically depicts in cross-section a variant or alternative embodiment of an optical fiber according to the present disclosure; FIG. 2 schematically depicts in cross-section a variant or alternative embodiment of an optical fiber according to the present disclosure; FIG. 5 illustrates a variation of a fiber amplifier that includes a VLMA single mode amplification fiber according to the present disclosure. 5 illustrates a variation of a fiber amplifier that includes a VLMA single mode amplification fiber according to the present disclosure. 1 illustrates a fiber laser system including a VLMA single mode amplification fiber according to the present disclosure. 1 illustrates a core-pumped fiber amplifier including a VLMA single-mode amplification fiber according to the present disclosure.

Apparatus The present application proposes a step index super large mode area fiber 100 having a core 1 doped with ions providing at least one emission band and at least one first cladding 2 surrounding the core 1. Core 1 extends along the longitudinal axis 10 of the optical fiber. Generally, rare earth ions are added to the core 1. For example, the rare earth ions are selected from all lanthanide ions. Preferably, the rare earth ions are selected from ytterbium, erbium, thulium, and holmium, or any combination thereof, such as erbium-ytterbium co-doping. Alternatively, core 1 is doped with chromium or bismuth ions. In the example below, the core 1 is based on a silica matrix doped with yttrium ions.

Core 1 is solid. Core 1 generally has a generally stepped refractive index profile relative to the first cladding. For example, core 1 has a flat top or parabolic index profile. The core has a generally cylindrical shape of circular cross section. The center of the core merges with the longitudinal axis 10 of the optical fiber.

The first cladding 2 comprises a solid matrix made of a first glass and comprising two stress-applying parts (i.e. SAPs) 21 , 22 . The two stress applying parts 21 and 22 are arranged symmetrically with respect to the core 1 inside the solid base material of the first cladding 2. In the cross-sectional plane (the plane of FIG. 1), the optical fiber 100 provides an alignment axis 20 passing through the core 1 and the two stressors 21, 22. The first cladding 2 does not contain any holes or other inclusions, except for the two stress-applying parts 21, 22. In other words, the first cladding 2 does not have any holes, ie it does not have a porous structure. The first cladding 2 is therefore entirely solid.

In the first embodiment shown in FIG. 1, each of the two stress applying parts 21, 22 has a cylindrical shape with a circular or disc-shaped cross section. Consider the center 11 or 12 of the stress applying portion 21 or 22. Centers 11 and 12 are aligned to an accuracy of a few degrees along an alignment axis 20 passing through the longitudinal axis 10 of the fiber. The longitudinal axis 10 is perpendicular to the plane of FIG. This design forms a panda-shaped polarization-maintaining fiber.

Alternatively, the SAPs have other shapes, such as a sector, and are placed symmetrically with respect to the core 2 to form a bowtie-shaped polarization maintaining fiber.

In the first embodiment shown in FIG. 1, the first cladding 2 has a cylindrical shape with a non-circular cross section. More precisely, the first cladding 2 comprises two flat surfaces 4, 14. The flat surface 4 is connected to the other flat surface 14 by two curved surfaces 5, 15. Generally, each curved surface 5, 15 has the shape of a circular arc or a circumferential arc in the cut plane. Each flat surface 4 , 14 extends parallel to the longitudinal axis 10 of the fiber and transverse to the alignment axis 20 . Preferably, the two flat surfaces 4, 14 are parallel to each other and perpendicular to the alignment axis 20.

Due to the shape of the glass cladding with two flat surfaces 4, 14 joined by two curved surfaces and the hardness of the cladding material, in a plane transverse to the two flat surfaces 4, 14, more precisely two stress-applying sections. An optical fiber is obtained which has preferential coiling properties in a plane containing the alignment axis 20 of 21, 22 and the longitudinal axis 10 of the fiber. When bent in this way, the radius of curvature of the fiber crosses the flat surfaces 4,14. As used herein, the expression "a plane across a flat surface" means that the plane is inclined with respect to the alignment axis 20 by an angle θ of less than 20 degrees, preferably less than 15 degrees. More precisely, each turn of fiber 100 generally lies in a plane that makes an angle of less than 20 degrees with respect to alignment axis 20.

In one example, the first cladding 2 is made of silica glass (or quartz glass), for example based on a pure silica (SiO ₂ ) matrix. Alternatively, the first cladding 2 is made of a glass other than silicon oxide. For example, the first cladding 2 is made of fluoride glass (eg ZBLAN). In another example, the first cladding 2 is made of a chalcogenide glass, ie a glass containing one or more chalcogens such as sulfur, selenium and/or tellurium, but without oxygen. Also, in another example, the first cladding 2 is made of phosphate glass. The core 1 is made of the same type of glass as the first cladding 2, whether based on silicon oxide glass, fluoride glass, chalcogenide glass or phosphate glass, and the core 1 is further impregnated with active dopants. include.

The two stress applying parts 21, 22 are made of doped glass bars. For example, the two stress applying parts 21 and 22 are made of silica glass bars doped with boron trioxide (B ₂ O ₃ ). Alternatively, the stress applying parts 21 and 22 may be made of a co-dopant of aluminum oxide and aluminum trioxide (Al ₂ O ₃ -B ₂ O ₃ ) or a co-dopant of aluminum oxide and phosphorus pentoxide (Al ₂ O ₃ -P _{2 ).} The glass bar is doped with O ₅ ) or any combination of suitable dopants to form a stressed portion with a negative refractive index different from the first glass of the first cladding 2. As an example, the difference in refractive index between the stress applying parts 21, 22 and the first glass of the cladding 2 is on the order of -10.10 ^-3 . Optical fiber 100 is a polarization maintaining fiber.

FIG. 2A schematically illustrates a cutaway view of a refractive index profile of an optical fiber according to an example of the present disclosure. In this example, core 1 has a flat top refractive index profile. This fiber is also called a stepped index fiber. The core 1 has a higher refractive index than the base material of the first cladding 2. The stress applying parts 21 and 22 have a lower refractive index than the base material of the first cladding 2.

Fiber lasers and amplifiers based on single-mode VLMAs operating in the spectral range of 1000 nm. In this case, the difference in refractive index between the core 1 and the matrix of the first cladding 2 is greater than 5.10 ⁻⁴ , typically 5.10 ⁻ It is in the range of ⁴ to 1.10 ^-3 . This small difference in refractive index requires tight dopant control during fiber preform manufacturing. This range of refractive index differences allows the core to provide a numerical aperture of 0.038 to 0.054. For fiber lasers and amplifiers based on single-mode VLMAs operating in the 2000 nm spectral range, the maximum refractive index difference is of the order of 2×10 ⁻³ .

FIG. 2B schematically illustrates a cutaway view of the refractive index profile of an optical fiber according to another example of the present disclosure. In this example, the core has a parabolic index profile. The first cladding 2 and the stress applying parts 21, 22 are similar to those shown in FIG. 2A. Advantageously, the parabolic profile provides a fundamental mode that is less sensitive to bending due to less reduction in effective mode area.

According to a variant of the refractive index profile shown in FIG. 8A, the fiber core may include a pedestal 6 between the central part of the core 1 and the first cladding 2. The pedestal consists of a solid annular region surrounding the central part of the core and having a refractive index lower than the central part of the core 1 and higher than the matrix of the first cladding 2. The ratio of the pedestal diameter to the core diameter is on the order of at least 2. The pedestal can reduce the difference in refractive index between the core 1 and the first glass of the first cladding 2.

The first cladding may be uncoated. Alternatively, the first cladding is covered with an outer cladding 8 (see Figure 8B). The cladding 8 is for example made of a low refractive index polymer (compared to the refractive index of the first cladding 2) to form a double clad fiber with a numerical aperture greater than 0.35. The outer cladding 8 also has a lower hardness or a higher modulus than the first cladding 2. The outer cladding 8 may have a circular cross section. For example, the first cladding 2 is made of quartz glass with an elastic modulus (at 20° C.) of approximately 7.25×10 ⁴ N/mm ² . The diameter of the first cladding 2, designated 2a, is about 220 μm, and the length C of the flat surface is about 80-125 μm. The cladding 8 is a low refractive index primary coating made of polymer. The polymer exhibits an elastic modulus (at 20° C.) of 20 N/mm ² to 500 N/mm ² . The thickness of the outer cladding 8 is approximately 50 μm. For example, the polymer is manufactured by MY Polymers Ltd. OF-1375-A manufactured by. The stiffness of the first cladding 2 is therefore such that the optical fiber 100 can be bent in a bending plane 30 across the flat surfaces 4, 14, ie at an angle θ of less than 15 degrees. Since there are only two mutually parallel flat surfaces, the bending is preferably carried out in a plane transverse to the two flat surfaces. Therefore, it is easy to wind the fiber into a proper plane.

According to a second embodiment, the first cladding 2 is surrounded by a second cladding 3 comprising a matrix made of a second glass whose refractive index is lower than the first glass of the first cladding 2. (See Figure 8C). The second cladding 3 is doped with fluorine so that the difference in refractive index with the first glass is -26.0×10 ⁻³ at maximum, thereby forming an all-glass double clad fiber. In this case, the first cladding 2 may have a circular cross-section and the second cladding 3 has two flat surfaces 4, 14 extending transversely to the alignment axis of the stress-applying parts 21, 22. As disclosed with respect to the first embodiment, the two flat surfaces 4, 14 are joined by two curved surfaces 5, 15. The outer cladding 8 is then made of a high refractive index acrylate coating.

According to a variant of the second embodiment, the second cladding 3 comprises an air cladding 7 surrounding the first cladding 2. The air cladding 7 is embedded in a solid matrix 13 made of a second glass (see Figure 8D). The outer cladding 8 is then made of a high refractive index acrylate coating.

The second cladding 3 may be made of a second glass whose refractive index is lower than the first cladding 2 (FIG. 8C) or it may include an air cladding 7 surrounding the first cladding 2, the air cladding 7 being a second glass. When embedded in a solid matrix 13 made of second glass (FIG. 8D), an outer cladding made of thin metal can be formed. For example, the outer cladding is made of aluminum, copper, or gold with a maximum thickness of about 15 μm. Due to the relatively small thickness of the metal cladding relative to the fiber diameter (approximately 220 μm), the hardness of the second cladding 3 carrying the two flat surfaces 4, 14 is such that the optical fiber 100 is preferably , 14 in a bending plane 30 that intersects the bending plane 30 .

Therefore, the fiber can be all-solid (without air cladding) or of the air-hole fiber type (when air cladding 7 is between the first cladding 2 and the solid matrix 13 of the second cladding 3). .

As an option, a low index polymer cladding 8 is placed around the second cladding 3.

In one example according to the first embodiment, the fiber is flat-top index as shown in FIG. 2A. FIG. 4 schematically shows a diagram obtained from an SEM micrograph of a cross-section of a polarization-maintaining fiber. The core diameter of the fiber is approximately 43.6 μm and the first cladding diameter (2a) is 246 μm. The length C of the flat surfaces 4 and 14 at the cut surface is approximately 110 μm. The refractive index difference between the core and the first glass is approximately 7.0×10 ⁻⁴ . Boron is added to the stress applying parts 21 and 22, and the diameter thereof is 4 μm. The distance between the center of the core and the center of the stress applying section is approximately 65 μm. FIG. 3 shows the measured refractive index profile measured at 633 nm for the core and first cladding of this optical fiber. The refractive index of the core is about 1.4496±0.0001 and the refractive index of the first glass is about 1.4503±0.0001, therefore, the first glass of the core and the first cladding are The refractive index difference between them is about 7×10 ⁻⁴ . When operated at a wavelength of 1064 nm, this optical fiber has a very large mode area (VLMA) of approximately 790 μm ² and a mode field diameter of 31.75 μm. However, if this fiber is straight, it will not behave as a single mode fiber. Due to the large core diameter and small refractive index difference, propagation modes LP ₀₁ and LP ₁₁ can propagate within the core along a straight fiber. Nevertheless, the fiber is birefringent (ie, polarization-maintaining), thereby allowing the degeneracy of polarization modes along the x- and y-axes of the fiber. In particular, regarding the LP ₀₁ or LP ₁₁ modes, these are divided into LP _01x and LP _01y modes, or into LP _11xe , LP _11ye , LP _11x0 , and LP _11y0 modes.

According to the present disclosure, the optical fiber 100 is bent in a bending plane 30 tilted by an angle θ with respect to the alignment axis 20 of the SAPs. The trace of the bending plane in the cut plane of the fiber is also referred to as the bending axis or bending axis of the fiber coil.

FIG. 5 shows the total loss of propagation modes LP ₀₁ and LP ₁₁ for an optical fiber having the numerical characteristics described above and bent with a bend diameter of 18 cm. When the angle θ is between 40 and 90 degrees, the loss is higher than ~1 dB/m in all modes. The fiber operates in quasi-single mode and single polarization, which means that both the higher order mode (HOM, here LP ₁₁ ) and the y-polarized fundamental mode (FM, here LP _01y ) have losses of 10 dB/ This is because it is larger than m. However, the loss of the x-polarized fundamental mode (LP _01x ) is too high for practical use in fiber amplifiers or fiber lasers.

When the angle θ is less than 10 degrees, the loss of the LP _01x mode decreases to less than 0.1 dB/m and becomes negligible when the bending axis is aligned with the alignment axis 20 (in other words, when the angle θ becomes zero) become.

Single-mode operation of the optical fiber is optimal when the angle θ is less than 5 degrees, i.e. the loss of the higher order modes (HOM, here LP ₁₁ ) is greater than 10 dB/m, while the x-polarized fundamental mode The loss of (LP _01x ) is 0.05 dB/m.

FIG. 6 shows the total loss of propagating modes in the direction of the bending axis along the alignment axis of the SAPs, or when the angle θ is zero. Figure 7 shows the total loss of propagating modes in the direction of the bending axis perpendicular to the alignment axis of the SAPs, ie along the y-axis, or when the angle θ is 90 degrees.

In FIG. 6, when the optical fiber 100 is bent in a plane parallel to the alignment axis 20 of the fiber core and SAPs, the loss of the fundamental mode LP _01x remains low, here 0.05 dB, when the fiber bend diameter is less than 19 cm. /m, while the loss of higher order modes is greater than 10 dB/m. Therefore, this optical fiber naturally operates in single mode when bent in a plane parallel to the alignment axis of the stressor, with a bend radius in the range of less than 21 cm, or even better, less than 19 cm. do. In order to limit losses in the fundamental mode, especially for fibers with lengths of several meters, the bending diameter is preferably greater than 15 cm, preferably greater than 16 cm. For example, the bending diameter is 16 cm to 19 cm, preferably 17 cm to 18 cm.

On the other hand, as shown in FIG. 7, when the optical fiber 100 is bent in a plane perpendicular to the alignment axis 20 of the fiber core and SPAs, when the bending diameter is smaller than 20 cm, the fundamental mode loss increases, The loss of at least one of the higher order modes remains low (less than 1 dB/m) when the bend radius is greater than 0.1 dB/m and the bend radius is greater than 19 cm. Therefore, this optical fiber cannot operate in single mode when bent in a plane perpendicular to the alignment axis of the stressor.

The boron-doped stressors 21, 22 produce two technical effects when the fiber is bent in a plane inclined by an angle of less than 15 degrees with respect to the alignment axis of the stressor. First, the fundamental mode exhibits a higher confinement effect due to the presence of two boron-doped stress applying parts. Therefore, the loss of the fundamental mode (LP _01x ) is negligible when the fiber is wound or bent in a plane parallel to the alignment axis 20. Second, a portion of the higher-order mode electromagnetic field extends into the boron-doped stressor, which induces high losses. Therefore, higher order modes do not exhibit the same confinement effect as the fundamental mode of LP _01x . The optical fiber of the present disclosure, when bent in a plane across a flat surface, allows single mode operation with limited loss (less than 0.5 dB/m) in the fundamental mode.

As used herein, the degree of confinement refers to the proportion of the modes that are considered to be contained within a certain radius relative to the center of the fiber, and thus the center of the core. Modes that are properly confined within the core exhibit a degree of confinement close to 1, or approximately 100%.

Furthermore, the orientation of the two flat surfaces 4, 14 induces a preferential bending of the fiber with a radius of curvature parallel to the alignment axis 20 or x-axis when the fiber is placed in a plane. For example, a fiber is placed between two flat planes and the ends of the fiber are held such that their flat faces 4, 14 are oriented perpendicular to the flat planes. When winding up the fiber, the fiber naturally bends so that the alignment axis of the stressor remains parallel to the two flat planes.

Other examples of ultra-large mode area fibers according to the present disclosure have the following characteristics. The core diameter is 35 μm. The core is made of a silica matrix doped with ytterbium ions. The refractive index difference between the core and the first glass is about 7.3×10 ⁻⁴ . The first cladding includes two boron-doped stressors, each with a diameter of 48 μm. The center-to-center distance between the core and each stress applying portion is 62.5 μm. The fiber diameter is 220 μm. The length of the flat surface in the cutting plane is 110 μm. The flat surfaces 4, 14 are oriented in a plane transverse to the bending radius of the fiber. The bending diameter is 15 cm to 18 cm. When operating at a wavelength of 1064 nm, the effective area of this fiber is 615 μm ² , which corresponds to a mode field diameter of 28 μm. The loss in the higher order modes is higher than 10 dB/m, while the loss in the LP _01x fundamental mode remains below 0.1 dB/m.

The amplification fiber 100 generates amplified light of a certain wavelength depending on the element added to the core. When doped with ytterbium ions, the VLMA single mode amplification fiber 100 is adapted to amplify light at wavelengths in the range of 950 nm to 1150 nm. When doped with erbium ions, the VLMA single mode amplification fiber 100 is adapted to amplify light in the wavelength range of 1530 nm to 1610 nm. When doped with thulium ions, the VLMA single mode amplification fiber 100 is adapted to amplify light in the wavelength range of 1900 nm to 2100 nm. When doped with holmium ions, the VLMA single mode amplification fiber 100 is adapted to amplify light in the wavelength range of 1950 nm to 2160 nm. Those skilled in the art will readily be able to select the appropriate additive composition of the core depending on the desired operating wavelength range. Of course, the seed light source and excitation light source(s) are also adapted accordingly.

The present disclosure thus proposes an ultra-large mode area polarization maintaining and amplifying fiber that operates in the single mode regime with negligible losses in the fundamental mode. For example, polarization-maintaining fibers are panda-shaped. Preferably, the fiber core is doped with rare earth elements.

Such VLMA fibers can be used either in high peak power fiber amplifiers or in high power continuous wave or pulsed fiber lasers, while providing strictly single mode operation.

The fiber is bent with a bend diameter of 10 cm to 30 cm, which allows the use of fiber lengths from 50 cm to meters or tens of meters, while compacting the footprint in the low-loss single mode regime. Furthermore, if the fiber is of the step index type, it is easy to manufacture at low cost. The fibers can also be easily cut and spliced to other fibers, thereby allowing industrial scale manufacturing of all-fiber laser systems.

FIG. 9 shows a first example of a fiber amplifier 110 that includes a VLMA single-mode amplification optical fiber 100 as a forward-pumped type. VLMA single mode amplification optical fiber 100 comprises an optical fiber according to any of the disclosed embodiments. In the fiber amplifier 110, a continuous wave (CW) or pulsed laser is used as a seed light source that will be amplified by the VLMA single mode amplification optical fiber 100. The output of light source 9 is coupled to the input signal arm 19 of double clad excitation signal combiner 26 . Fiber amplifier 110 includes one or several pump light sources 25 for generating pump radiation adapted to optically pump the dopant elements of the core to amplify the seed signal. The pump signal combiner 26 connects on its input side a light source 9 and pump(s) 25 and on its output side on a first end of a VLMA single mode amplification optical fiber 100, e.g. a passive double-clad optical fiber 27. connected through a part of Beam combiner 26 combines the seed signal and the excitation beam. Therefore, a seed signal is injected into the core 1 of the optical fiber 100 and a pump beam is injected into the glass cladding of the fiber 100. An amplified signal is generated at the second end of the VLMA single mode amplification optical fiber 100.

FIG. 10 shows an alternative embodiment of a backward pumped fiber amplifier 110. Here, a seed light source 9 is connected to a first end of a VLMA single mode amplification optical fiber 100 using, for example, a conventional single mode fiber splice. A pulse signal combiner 26 is connected to the second end of the VLMA single mode fiber 100 on one side and to the pump(s) 25 and other fiber splices 24 on the other side. Amplified pulses are available at the output of fiber splice 24.

VLMA single mode amplification optical fiber 100 can also be used with fiber lasers. FIG. 11 schematically shows the structure of a laser fiber 120 based on fiber 100 according to any of the disclosed embodiments. A VLMA single mode amplification optical fiber 100 is installed in a cavity formed by two mirrors. For example, the cavities include a first fiber Bragg grating 28 installed at the first end of the VLMA single-mode amplification optical fiber 100 and a second fiber Bragg grating installed at the second end of the VLMA single-mode amplification optical fiber 100. 29. Alternatively, the cavity is formed by a bulk dielectric or metal mirror, and signal injection into the VLMA fiber is realized in free space. Fiber lasers using VLMA single mode amplification optical fiber 100 can be arranged in forward, backward, or bidirectionally pumped configurations.

VLMA single mode amplification optical fiber 100 may also be core pumped. FIG. 12 schematically shows the structure of a fiber amplifier 130 based on a fiber 100 according to any of the disclosed embodiments, where the optical fiber 100 is core pumped. In this configuration, the output of seed laser 9 and the single mode output of pump laser 32 are coupled into multiplexer 33 to input legs 30 and 31 of multiplexer 33, respectively. Multiplexer output leg 34 is spliced to the input end of VLMA single mode amplification optical fiber 100. Therefore, both the seed signal and the single mode pump are injected into the core 1 of the optical fiber 100. Advantageously, optical fiber 100 is a single-clad fiber (eg, as shown in FIGS. 1, 4, or 8A). An amplified signal is generated at the second end of the VLMA single mode amplification optical fiber 100.

While representative examples of VLMA single amplification optical fibers, fiber amplifiers, and fiber lasers are described in detail herein, those skilled in the art will appreciate that the scope of this disclosure is defined by the appended claims. It will be appreciated that various substitutions and improvements may be made without departing from the invention.

Claims (15)

An ultra-large mode area single mode amplification optical fiber (100) comprising a core (1) extending along the longitudinal axis (10) of said optical fiber (100), said core being solid, said core comprising at least The core (1) is doped with an element providing one emission band, the core diameter of the core (1) is greater than 30 micrometers, the core (1) is coated with at least one glassy cladding (2), including a first cladding (2). , 3), the first cladding (2) is surrounded by a solid matrix made of a first glass, and two stress applying parts (21, 22) arranged symmetrically with respect to the core (1). the first glass has a lower refractive index than the core, and the core (1) and the two stress-applying parts (21, 22) have an alignment axis (20) transverse to the longitudinal axis (10). ), said at least one glassy cladding (2, 3) extending at its outer periphery parallel to said longitudinal axis (10) and transverse to said alignment axis (20). comprising two flat surfaces (4, 14), said two flat surfaces (4, 14) arranged symmetrically with respect to said core (1) and coupled by two curved surfaces (5, 15), said optical fiber ( 100) is suitable to be bent with a bending radius of less than 30 cm in a plane (30) containing said longitudinal axis (10) of said fiber, said plane (30) being aligned with an alignment axis (20) and 15 An ultra-large mode area single mode amplifying optical fiber (100) having a bending loss of less than 0.5 dB/m in the fundamental mode at an angle of less than 100 degrees. The ultra-large mode area single mode amplification optical fiber according to claim 1, wherein the bending diameter is between 10 cm and 20 cm, and the bending loss is less than 0.5 dB/m in the fundamental mode. Ultra large mode area single mode amplifying optical fiber according to claim 1 or claim 2, wherein the optical fiber (100) is bent in a plane (30) making an angle of less than 10 degrees with the alignment axis (20). Fiber amplifier (110) according to any of the preceding claims, wherein the ultra-large mode area single mode amplification optical fiber (100) provides an effective area of greater than 450 μm ² . Super large mode according to any one of claims 1 to 4, wherein the core (1) and the first glass are based on silica glass, or on fluoride glass, or on chalcogenide glass, or on phosphate glass. Area single mode amplification optical fiber. The ultra-large mode area single mode amplification optical fiber according to any one of claims 1 to 5, wherein the core (1) is doped with rare earth ions or chromium ions. The core (1) exhibits a flat-top or parabolic refractive index profile, or the core (1) includes a pedestal (6) surrounding a central part of the core (1), the pedestal (6) 7. The ultra-large mode area single mode amplifying optical fiber according to claim 1, wherein the ultra-large mode area single mode amplifying optical fiber has a refractive index lower than the central portion of the glass and higher than the first glass. 2. The at least one glassy cladding (2, 3) consisting of a first cladding (2), said first cladding comprising on its outer circumference said two flat surfaces (4, 14). 8. The super large mode area single mode amplification optical fiber according to any one of items 1 to 7. Said at least one glassy cladding (2, 3) comprises a second cladding (3) arranged around said first cladding (2), said second cladding (3) The ultra-large mode area single mode amplifying optical fiber according to any one of claims 1 to 7, having a refractive index lower than that of glass. The second cladding (3) is selected from an all-solid cladding (3) made of a second glass or an air cladding (7) and a solid cladding (13) made of a second glass; (7) is arranged between the first cladding (2) and the solid cladding (13) made of the second glass, an ultra-large mode area single mode amplification according to claim 9 optical fiber. Ultra-large mode area single mode amplifying optical fiber according to any of the preceding claims, further comprising a polymer or metal cladding (8) around the at least one glassy cladding (2, 3). A fiber amplifier (110) comprising a super large mode area single mode amplifying optical fiber (100) according to any one of claims 1 to 11, wherein the super large mode area single mode amplifying optical fiber (100) is less than 30 cm. a fiber amplifier (110) wound with a bending diameter of . Fiber amplifier (110) according to claim 12, wherein the ultra-large mode area single mode amplification optical fiber (100) has a length of 50 cm to 20 m. an excitation light source (25) for generating an excitation beam and an optical beam combiner (26) adapted to inject said excitation beam into said core (1) and/or into said first cladding (2). A fiber amplifier (110) according to any one of claims 12 to 13, comprising: A fiber laser (120) comprising a super large mode area single mode amplifying optical fiber (100) according to any one of claims 1 to 11, wherein the super large mode area single mode amplifying optical fiber (100) is less than 30 cm. a first mirror (28) at a first end of the ultra-large mode area single mode amplification optical fiber (100); a second mirror (29) at a second end of the ultra-large mode area single mode amplification optical fiber (100).

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